Abstract

Global warming is predicted to result in a significant shallowing of the mixed layer depth (MLD) in many ocean regions and, thus, warmer surface waters (within the MLD), which could have dire consequences for coral reef ecosystems and the ocean’s capability for CO₂ uptake. The seasonal to long-term variability of the MLD within the Coral Sea was examined using conductivity-temperature-depth (CTD) profiles from the World Ocean Database (WOD), and of Argo floats deposited within the Coral Sea or its vicinity since July 2001. A distinct seasonality in the MLD is evident throughout the Coral Sea, but was generally more pronounced in higher latitudes as a result of greater seasonality in sea surface temperature and wind stress. While summer mixed layers are relatively homogeneously shallow throughout the Coral Sea, winter mixed layers in higher latitudes tend to be significantly deeper compared to tropical regions. No long-term trends in mean monthly MLD are evident, possibly due to the relative data scarcity prior to the onset of the Argo project. The Argo program represents a significant advancement for marine studies. In the future, it will enable more detailed studies on long-term variability and trends of the MLD and its associated impact on the photic zone and the Great Barrier Reef.

Due to the absence of a similar program for ocean biogeochemistry, seasonal and long-term changes in fugacity of CO₂ (fCO₂) and pH within the Coral Sea were investigated not only using observational data (from WOD and other sources), but also with the sophisticated coupled regional model ROMS-PISCES. Both observational fCO₂ and pH exhibit some seasonality in the Coral Sea. Seawater fCO₂ (fCO₂sea) values tend to increase towards summer, which is linked to the positive relationship between fugacity and sea surface temperature. As a result, the capacity of the Coral Sea to act as a sink of atmospheric CO₂ is reduced during summer, and is more likely to act as a source of CO₂ to the atmosphere. No long-term changes were discernible in the very sporadically observed pH values, whereas fCO₂sea appears to have increased at a rate of 1.41 ± 0.04 μatm/year from 1983 to 2001.

As a result of increased atmospheric CO₂ levels, oceanic geochemistry has already significantly changed since 1880. The effects of higher atmospheric fCO₂ will likely be further aggravated by shallower MLDs as a result of reduced upper ocean mixing due to warmer sea surface temperatures. Various IPCC scenarios for predicted atmospheric CO₂ levels were used to determine likely changes in the geochemistry of the Coral Sea during the 21st century. Projected increases of atmospheric CO₂ to 650-1000 ppmv results in a decrease of sea surface pH by 0.14-0.38 units in the numerical model, with the Coral Sea simultaneously changing from a seasonal source of atmospheric CO₂ to a predominant sink. Concurrent with increased ocean acidification and fCO₂, the saturation state of aragonite and calcite will decline significantly, which would have wide-reaching effects on the coral calcification rates and the general health, and structural strength, of calcifying organisms. These biogeochemical effects will be exacerbated by an expected decrease in the MLD throughout the Coral Sea, and concomitant higher temperatures within the mixed layer. To this date, there has been surprisingly little effort to monitor the changes in biogeochemistry within the Coral Sea and, specifically, within the GBR as a result of increased atmospheric fCO₂. Further large-scale studies are required throughout the entire Coral Sea in order to accurately determine the long-term trends in the oceanic carbon cycle.

Item ID:

26651

Item Type:

Thesis
(PhD)

Additional Information:

Publications arising from this thesis are available from the Related URLs field. The publications are:

Jaffrés, J., and Everingham, Y. (2005) An exploratory investigation of the relationship between decadal rainfall and climate indices. Proceedings of the 2005 Conference of the Australian Society of Sugar Cane Technologists. 27th Annual Conference Australian Society of Sugar Cane Technologists , 3-6 May 2005, Bundaberg, QLD, Australia , pp. 96-108